1. Field of the Invention
The present invention relates to a technology of a dual motor drive system, and more particularly to a dual motor drive system for driving two motors individually by a single inverter circuit.
2. Description of the Related Art
Generally, two synchronous motors need to be provided with individual inverters in order to drive the motors individually (by different phases and at different rotating speeds). The inverter is designed for its current capacity to have a maximum current (a maximum torque) flow through the motor to be driven. In a system having the synchronous motor, however, there are relatively few times when the motor generates a maximum torque during operation of the system. Usually, the motor requires the maximum torque during a certain period of time, and during the rest of the time the motor being driven requires a lower torque.
For example, a drive motor of an electric automobile requires a maximum torque when accelerating from a low speed and starting to move up on a steep gradient, but during the rest of the time the motor being driven requires less than the maximum torque. It is assumed that an inverter usage ratio is determined by (an amount of passed current×a time to pass the current)/(a maximum amount of current×a system operating time). Because a torque during the normal driving state is several to several dozen percents of a maximum torque, the inverter usage ratio usually is determined to be several to several dozen percents. Thus, although the inverter is designed to generate a maximum current corresponding to a maximum torque, the inverter generates the maximum current for a certain period of time, and during the rest of the time the inverter is operated with a large amount of current to spare.
An electric automobile is equipped with various motors for accessories such as an air conditioner and a power steering, as well as the drive motor. In a system having a plurality of synchronous motors, each of the synchronous motors needs to be provided with an inverter used for itself, although the inverter has low usage ratio, thereby causing problems of cost and space for mounting the inverters.
There have been known techniques for driving two motors by a single inverter. For example, Japanese Laid-open Patent Application No. 2001-103717 (paragraphs [0033, 0034], FIG. 2) discloses combined motors having plural rotors. Each of the combined motors has a stator coil and a rotor separately (the combined motors may be incorporated in one, or arranged separately). In the combined motors, the same magnetic poles of the two stator coils are connected to each other in parallel, and combined currents pass through the coils from a single inverter. Thereby, each of the two rotors is driven individually at any rotating speed.
With the above-described configuration, however, because the two coils are connected to each other in parallel, there may be a problem that a current component of one of the coils passes through the other coil and more ineffectual current is generated through the coils, thereby increasing copper loss or the like.
In consideration of the problem as described above, Japanese Laid-open Patent Application No. 2001-231227 has proposed combined motors for reducing the generation of an ineffectual current generated when the same magnetic poles of two stator coils are connected to each other in parallel, respectively, so as to improve the driving efficiency of the motors.
However, the technique disclosed in the above-described Japanese Laid-open Patent Application No. 2001-103717 has the serious problem of the ineffective current generated when the combined currents flow into the two stator coils. Also, in the above-described Japanese Laid-open Patent Application No. 2001-231227, the three-phase motor includes a motor having a rotor with equal to or more than three pairs of magnetic poles, and a three-phase motor having less than or equal to two pairs of magnetic poles may not be applied.